Calculating the 'twistiness' (torsion?) of the line/curve through a set of points

Hello, this is programmer finding himself in need of a math-based algorithm.. hopefully this is a reasonable place to ask :)

The data is a set of latitude/longitude (X,Y) points recorded by a GPS device.

I want to determine the "twistiness" (torsion? curvature?) of a curve that follows all of those points...

A result of some coefficient (0-1, perhaps? 0 being straight 1 being.. hm, really twisty?) would be ideal.

Any advice would be great, it's been a while since I've had to do anything like this. Not really sure where to start of what resources to search for. Thanks!